1. Field of the Invention
The present invention relates to a solar blackbody waveguide that captures and uses sunlight to heat a thermal working fluid, such as compressed air. Solar cell arrays are used to capture the sunlight. The arrays are mounted on a plurality of solar towers so that the solar cell arrays can be moved to track the daily movement of the sun and can be tilted to maintain the proper orientation as the angle of the sun with the horizon changes throughout the day and the year. The solar cell arrays direct the light rays into a series of light pipes, optical tees, optical elbows, optical reducers, and solar horns to deliver the light rays into a solar coil located within an underground insulated pipeline that is provided only a few feet away from the base of the solar towers. Energy from the light rays is absorbed by the solar coil and transferred to the pressurized thermal working fluid that flows through an annular space provided between the solar coil and the insulated pipeline. The energy laden thermal working fluid is removed from the annular space at a downstream end of the insulated pipeline so that it can be used with existing technologies, such as with a combined cycle gas turbine. The invention can be used in association with any type of commercial or industrial installation requiring hot water, steam or heat.
Once the thermal working fluid is heated, it can then be used in association with existing technologies. For example, the present invention can be used as an air preheater to heat air for use in association with a combined cycle gas turbine. If water is used as the thermal working fluid, the present invention could be used as a steam generator for use in steam cycle turbine plants or other commercial or industrial processes requiring steam.
2. Description of the Related Art
The effective use of solar flux as a source of heat to drive heat engines has been the aim of numerous thermal-solar energy technologies. Unlike photoelectric solar cells which convert solar energy directly into electrical energy, thermal-solar technologies convert solar energy into heat which is then converted into mechanical energy and finally into electrical energy. Typically, at the center of this conversion process in current technologies is the steam or Rankine cycle.
Low cost production of electricity using current steam cycle technologies is based on magnitude-of-scale production. Production of electricity in the Megawatt (MW) range requires enormous amounts of heat. Assembling enough energy from weak solar energy in a single location to power a generator in this range remains the defining technical challenge of this form of solar energy.
The low energy-density of ambient sunlight requires that the geometry of concentrator assemblies be very large. Assembling enough energy in one location to power a large heat engine has been handled by three primary methods. The first method uses thermal transfer fluid to accumulate heat as it passes from one incremental heat generator to another. The second method transmits large quantities of solar energy over large distances in a nearly lossless manner to a single “receiver” point. The third method generates electricity using small generator systems and the total produced power is then assembled via a distributed electrical bus.
Systems involving each of all of these three methodologies have been developed to the point of operation. However, each system has introduced its own technical complications, thermal losses, and inefficiencies as described briefly below.
Several trough systems were built in the mid to late 1980's. One such system was the parabolic trough system. This type of system incrementally accumulates energy by using a heat transfer fluid. Sunlight is focused using a parabolic trough-shaped mirror on to a pipe containing a heat transfer fluid, typically thermal oil. This hot oil is passed successively through a number of parabolic trough concentrators until the temperature of the oil is heated to approximately 390° C. (735° F.). This hot oil is then passed through a heat exchanger to generate superheated steam from which electricity is generated using a conventional steam turbine.
The parabolic trough system has several drawbacks. This system has high thermal losses due to the fact that the oil-filled pipe at the center of the concentrator trough is not insulated and re-radiates the accumulated heat back into space. Also, not all the solar energy incident on the pipe containing the heat transfer fluid is absorbed by the fluid. In fact, most of the energy is reflected. In addition, use of a heat exchanger in the steam generator loop increases the overall inefficiencies of the system. These components combine to limit the gains that can be acquired from magnitude-of-scale operation. In addition, there are other limitations for these implementations since these systems do not track the sun from east to west, although they do track the seasonal inclination angle. As a result, they are typically constructed with a “due-south” orientation and are most effective in the late morning to early afternoon.
At least two power tower systems were built in the mid 1980's to mid 1990's. This type of system concentrates sunlight over a large area by transmitting it in a lossless manner through ambient air to a receiver point located at the top of a power tower. Mirrors or heliostats are mounted on the ground surrounding the power tower. These heliostats track the sun and reflect the light from the sun up to the power tower where a thermal fluid system is located. The power tower is in essence a large, fragmented collector dish distributed over a large area. The heat transfer fluid is molten sodium which is heated to approximately to 570° C. (1050° F.) as it passes through the receiver at the focal point of the power tower. This thermal fluid is then passed through a heat exchanger to generate superheated steam from which electricity is generated using a conventional steam turbine.
The power tower system also has several drawbacks. This system also has high thermal losses. With the receiver suspended in the air with limited insulation, it re-radiates accumulated heat back into space. Additionally, use of a heat exchanger in the steam generator loop increases the overall inefficiencies of the system. These thermal considerations combine to limit the gains that can be acquired from magnitude-of-scale construction and lower the overall thermal efficiency. In addition, there are other limitations for power towers since self-shadowing of the heliostats keeps them from providing power over the entire day.
Dish engine systems are in the advanced prototype phase with test facilities deployed in the late 1990's. These systems use an array of parabolic dish-shaped mirrors to focus solar flux to a small “receiver” located at the focal point of the parabolic mirror assembly. A thermal working fluid of water is heated to about 750° C. (1380° F.) and used directly to generate electricity using a small steam turbine attached to the dish without use of a heat exchanger. The electricity generated is collected using a system of electrical buses or collection systems for final connection to the utility electric grid. Due to the higher operating temperatures and elimination of heat exchangers, these systems have higher thermal efficiencies than parabolic troughs and power towers.
However, these dish engine systems do not overcome the same basic drawback of the other technologies, i.e. high thermal losses with the receiver suspended in the air with limited insulation and resultant with re-radiation of accumulated heat back into space. These systems can track the daily progress of the sun, and therefore, provide power for longer periods during the day. The addition of steam turbines attached to the dish generator increases the structural load-bearing requirements of the support system. The structures required to support the dish and engine can become massive and expensive to construct.
The current art in solar-thermal energy recognizes the need to effectively accumulate the necessary amounts of heat for magnitude-of-scale production. However, the means for doing this as demonstrated in the art is not entirely physically realizable. An example is shown in U.S. Pat. No. 4,982,723 where solar energy is introduced into a thermal fluid inducing a photochemical reaction. In this process, the need for accumulation of energy is recognized, but the physical mechanism for making it happen on a large scale is sketchy.
Another technology is reflected in U.S. Pat. No. 4,841,946 in which a Cassegrain reflector is used to concentrate solar flux. This concentrated flux is transported via light pipe to a cavity where the solar energy is converted into heat energy. Although this patent contains some interesting abstract concepts, it does not obtain a complete solution when scaled up for actual power production levels. This proposed technology implies (without a realizable solution) that if more energy is required, the energy from a plurality of similarly situated reflectors can be conducted to a single receiver via a plurality of light pipes. An inherent difficultly that this technology fails to overcome is the transmission losses associated with moving highly concentrated solar energy via light pipes over long distances. Although these losses are small, they are not zero, and a transmission loss of one tenth of one percent (0.1%) in a light pipe carrying one (1) MW of energy is an enormous amount of energy to be absorbed by the material from which the light pipe is constructed. Nor does this technology propose any useful method for combining light from the plurality of light pipes into a single, common light pipe for long distance transmission.